The breeding ratio is a key performance metric for fast-spectrum nuclear reactors. In a conventional thermal reactor, more fissile atoms are consumed than created, so the reactor gradually depletes its fuel. A breeder reactor with a breeding ratio above 1.0 actually produces more fissile material (typically plutonium-239 from uranium-238 or uranium-233 from thorium-232) than it consumes, effectively manufacturing new fuel during normal operation. This transforms the world's vast supplies of uranium-238 and thorium — otherwise largely useless for energy — into potential fuel.
The implications for energy sustainability are profound. Current thermal reactors use less than 1% of mined uranium. Fast breeder reactors could theoretically increase this to 60% or more, extending the world's uranium resources from centuries to millennia. The world's existing stockpiles of depleted uranium alone could power civilization for hundreds of years in breeder reactors. This was the original motivation for sodium-cooled fast reactor programs in the United States, France, Russia, and Japan during the latter half of the 20th century.
However, breeding also raises proliferation concerns, as the plutonium produced could theoretically be diverted for weapons purposes. This concern, along with historically lower uranium prices that reduced the economic urgency of breeding, slowed breeder reactor development in Western countries. Modern fast reactor designs like TerraPower's Natrium and GE Hitachi's PRISM are designed to operate flexibly — they can breed fuel, break even, or burn excess fissile material and nuclear waste depending on how they are fueled and configured. For deeper coverage, see DeepTechIntel's nuclear section.